Site-selection criteria for the Einstein Telescope Florian Amann1;2 and Fabio Bonsignorio3 and Tomasz Bulik4 and Henk Jan Bulten5;6 and Stefano Cuccuru7;8 and Alain Dassargues9 and Riccardo DeSalvo10;11 and Edit Fenyvesi12;13;14 and Francesco Fidecaro15;16 and Irene Fiori17 and Carlo Giunchi18 and Aniello Grado19;20 and Jan Harms21;22 and Soumen Koley5 and László Kovács23 and Giovanni Losurdo16 and Vuk Mandic24 and Patrick Meyers25 and Luca Naticchioni26;27 and Frédéric Nguyen28 and Giacomo Oggiano7;8 and Marco Olivieri29 and Federico Paoletti16 and Andrea Paoli17 and Wolfango Plastino30;31 and Massimiliano Razzano15;16 and Paolo Ruggi17 and Gilberto Saccorotti18 and Alicia M Sintes32 and László Somlai12;33 and Peter Ván12;34 and Matyas Vasúth12 1Department of Earth Sciences, ETH Zurich, Zurich, Switzerland 2Chair of Engineering Geology, RWTH Aachen, Aachen, Germany 3Heron Robots srl, I-16121 Genova, Italy 4Astronomical Observatory Warsaw University, 00-478 Warsaw, Poland 5Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands 6VU University Amsterdam, 1081 HV Amsterdam, The Netherlands 7Dipartimento di Chimica e Farmacia, Università degli Studi di Sassari, 07100, Sassari, Italy 8INFN Laboratori Nazionali del Sud, 95125 Catania, Italy 9Hydrogeology and Environmental Geology, Urban & Environmental Engineering (UEE), University of Liège, Belgium 10Riclab LLC, 1650 Casa Grande Street, Pasadena, CA 91104, USA 11University of Sannio at Benevento, Benevento I-82100, Italy 12Wigner Research Centre for Physics, Institute of Particle and Nuclear Physics, 1121 Budapest, Konkoly Thege Miklós út 29-33 13Institute for Nuclear Research (Atomki), Bem tér 18/c, H-4026 Debrecen, Hungary 14University of Debrecen, Doctoral School of Physics, Bem tér 18/b, H-4026 Debrecen, Hungary 15Università di Pisa, I-56127 Pisa, Italy 16INFN, Sezione di Pisa, I-56127 Pisa, Italy 17European Gravitational Observatory (EGO), I-56021 Cascina, Pisa, Italy 18Istituto Nazionale di Geosica e Vulcanologia (INGV), Sezione Pisa, Pisa, Italy 19INAF, Osservatorio Astronomico di Capodimonte, I-80131 Napoli, Italy 20INFN, Sezione di Napoli, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy 21Gran Sasso Science Institute (GSSI), I-67100 L’Aquila, Italy 22INFN, Laboratori Nazionali del Gran Sasso, I-67100 Assergi, Italy 23RockStudy Ltd, Pécs, Hungary 24University of Minnesota, Minneapolis, MN 55455, USA 25OzGrav, University of Melbourne, Parkville, Victoria 3010, Australia 26Università di Roma “La Sapienza”, I-00185 Roma, Italy 27INFN, Sezione di Roma, I-00185 Roma, Italy 28Applied Geophysics, Urban & Environmental Engineering (UEE), University of Liège, Belgium 29Istituto Nazionale di Geosica e Vulcanologia (INGV), Sezione Bologna, Bologna, Italy 30Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, I-00146 Roma, Italy 31INFN, Sezione di Roma Tre, I-00146 Roma, Italy 32Universitat de les Illes Balears, IAC3—IEEC, E-07122 Palma de Mallorca, Spain 33Institute of Physics Faculty of Sciences, University of Pécs, H-7624 Pécs, Ifjúság str. 6 and 34Budapest University of Technology and Economics, Faculty of Mechanical Engineering, Department of Energy Engineering, Budapest, Hungary The Einstein Telescope (ET) is a proposed next-generation, underground gravitational-wave (GW) detector to be based in Europe. It will provide about an order of magnitude sensitivity increase with respect to currently operating detectors, and furthermore, extend the observation band towards lower frequencies, i.e., down to about 3 Hz. One of the rst decisions that needs to be made is about the future ET site following an in-depth site characterization. Site evaluation and selection is a complicated process, which takes into account science, nancial, political, and socio-economic criteria. In this paper, we provide an overview of the site-selection criteria for ET, provide a formalism to evaluate the direct impact of environmental noise on ET sensitivity, and outline the necessary elements of a site-characterization campaign. arXiv:2003.03434v2 [physics.ins-det] 14 Jun 2020 I. INTRODUCTION Modern particle detectors are located underground to reduce the natural background. Sites of new ground-based tele- The environment surrounding modern fundamental scopes have to be chosen carefully to enable excellent seeing physics experiments assumes an increasingly important role conditions and to avoid light pollution [7–11]. Sometimes, with great impact on infrastructure, cost, and science. In the environment can even form an essential component of experiments to search for rare particle interactions like the the experiment itself like in large-scale neutrino detectors neutrino-less double-beta decay or interactions with dark [12, 13]. Even at CERN, where the direct impact of the matter, the local radioactive environment and particle back- environment can be corrected by feedback and plays a minor grounds can limit the sensitivity of the experiments [1–6]. role, environment-dependent aspects of infrastructure 2 lifetime are of great importance and need to be analyzed this article. Instead, we provide a summary of the respective [14]. Site characterization and selection is therefore of great site properties that will have to be studied for site selection. value in large modern fundamental-physics experiments and We limit the quantitative analysis to aspects that have a di- can crucially inuence their future scientic output. rect impact on ET’s sensitivity, i.e., the calculation of envi- The environment plays an even more important role for ronmental noise, neglecting relations that exist between all gravitational-wave (GW) detectors. For the LIGO and Virgo criteria due to nancial constraints. detectors, the site conditions were assessed especially with In section II, we discuss general site conditions related to respect to the feasibility of the construction, but also the im- geology, ground water, etc. In section III, we describe envi- portance of having an environment with weak seismic distur- ronmental noises and how to estimate associated ET instru- bances was emphasized [15, 16]. Ground motion, sound, and ment noise. Since site characterization plays such an impor- other environmental noises can directly aect the sensitivity tant role, we summarize the targets of a site-characterization and duty cycle of a GW detector [17]. For Einstein Telescope campaign in section IV and how to obtain the required infor- (ET), general site conditions concerning, for example, geol- mation. ogy and ground water can have a great impact on construc- tion cost, infrastructure lifetime, and environmental noise. A preliminary seismic assessment of numerous sites in Europe II. SITE CONDITIONS was carried out as part of the ET Conceptional Design Study [18, 19]. One of the goals of ET is to extend the frequency In this section, we discuss the site-selection criteria from band of ground-based GW observations down to a few Hertz an infrastructural and geological point of view. This should [20], which amplies the importance of environmental noise. include all the possible parameters that have an impact on the Seismic elds were given special attention since the main en- excavation costs and construction timeline, detector opera- vironmental noise predicted to set a low-frequency limit to tion, underground facility access convenience, safety of the ET’s bandwidth is from gravity perturbations produced by workers in the underground environment and detector life- seismic elds [21, 22]. Among the environmental noises, ter- time that we assume to be at least 50 years. The parameters restrial gravity perturbations, if they limit the detector sen- related to the underground facilities have been grouped in sitivity, require a complicated mitigation method [22]. Sup- terms of geological conditions, hydrogeological conditions, pressing terrestrial gravity perturbations is the main motiva- and geotechnical conditions. Another section concerns sur- tion to construct ET underground and therefore determines face conditions, infrastructures, and societal aspects. a large fraction of the cost. The main goal of site selection, site characterization, facil- Two candidate sites were chosen to be subject to a detailed ity layout, and identication of applied construction meth- site-characterization: north of Lula in Sardinia (Italy), and the ods is to nd a location that allows for the construction of Meuse-Rhine Euroregion. It is the responsibility of the ET ET so that it can achieve its science goals and operate eec- collaboration to present an evaluation of the two sites. A site tively for its proposed lifetime. The technical and cost as- evaluation needs to consider the impact of site conditions on: pects, nevertheless, can only be optimized together, as a re- sult of a multi-component decision-making procedure, bal- • Detector sensitivity ancing among sensitivity, cost and technical risk analyses. • Detector operation and duty cycle The most reasonable solution for the selected site, the ba- • Infrastructure lifetime sic design and the planned construction methods should en- sure optimization both for technical readiness and the overall • Site-quality preservation costs (both for construction and operation phases) of the fa- • Construction and maintenance cost cility. • Socio-economic impact of ET Individual environmental properties such as local geology, A. Geological conditions topography, and seismic activity can be relevant to more than one of these criteria. While it is helpful to introduce these The challenges